1. Field of the Invention
This invention relates to drive mechanisms and, more particularly, to drive mechanisms for converting a rotary motion into a linear force in a particular direction.
2. Description of the Prior Art
It is quite common in the mechanical arts to convert a rotary motion into a linear force. For example, in the pile driving area, it is desirable to translate a rotary force into a downward force which drives the pile into the ground. In the simplest version of a pile driver, a large weight is lifted vertically and then dropped onto the pile to drive it into the ground.
During the last 25 years, vibratory driver/extractors have come into wide use in the pile driving industry. The vibratory force is generated by one or more pairs of identical eccentrics having parallel axes of rotation. The pair of eccentrics rotate in opposite directions and are generally connected by gears so that they rotate in synchrony at the same speed. Such eccentrics are typically rotated by an electric motor or hydraulic drive unit.
These vibratory driver/extractors are primarily used on non-displacement piles, such as steel sheet piles, H-piles, open-end pipe piles and caissons. Conventional vibratory driver/extractors generate very high driving forces, from twenty to several hundred tons, but the force is actually reversed twenty to thirty times per second and so it does not really drive or extract the pile. A vibrating frame clamps onto the pile with a hydraulic clamp and the pile is vibrated up and down, generally on the order of 1/4-3/4 of an inch, and at a frequency between 1,000- 2,000 cpm. This vibration breaks the frictional bond between the pile and the soil. The weight of the apparatus causes the pile to penetrate into the soil overcoming only the point resistance which is small on a non-displacement pile. In extraction, the vibration breaks the soil friction and the pile is lifted out of the ground by raising the extractor with a crane hammer line.
Vibratory apparatus may also be used to compact soil and other material.
Conventional vibratory driver/extractors rotate the eccentrics and transmit the driving force through axles mounted on roller bearings. Because of the extremely high stresses in these bearings, they are short-lived and a constant source of breakdowns. Vibratory driver/extractors currently in use typically generate a driving force in one direction, and then generate a fraction of a second later an equal force in the opposite direction.
It is an object of this invention to provide a machine which will develop, in a simple and durable arrangement, a near constant force in one direction and little or no force in the opposite direction. It is also an object of the present invention to develop such a device which will have particular utility in driving piles, but will also have other applications where generating a unidirectional force is desirable.